Techniques have been known that determine road surface conditions from crack percentages of the road surfaces for maintenance and management of the road surfaces. In such techniques, a plurality of inspection areas each having a certain size (e.g., 50 cm×50 cm) are set on a road surface and the crack percentage is calculated for each inspection area from the number of cracks in the inspection area. A higher crack percentage indicates the road surface that has been further deteriorated.
JP-A 2008-46065 (KOKAI) discloses, for example, the following technique: for causing an inspector to know cracks on a road surface, a line camera or an area camera is put in a measuring vehicle such that the camera looks down the road surface, lighting is put in the measuring vehicle such that the lighting illuminates an imaging position of the line camera or the area camera, the illuminated imaging position is imaged by the line camera or the area camera while the measuring vehicle runs on the road, and an image of the road surface viewed from directly above the road surface is generated by composing the taken images.
The conventional technique, however, causes a difference in brightness between the taken images due to various factors such as brightness of the road surface and uneven brightness in the image. In the composed image obtained by composing the taken images, brightness changes at the border between the taken images in the composed image. As a result, the existence of such a border in the inspection area may cause the border to be erroneously recognized as abnormality.